The invention relates to an up-set shrinker for reducing and processing an O-shaped pipe-blank in production of steel pipe, particularly thick steel pipe in which the ratio of thickness/outer diameter of the pipe is greater than 2%.
The UOE process is known as one process for producing thick welded steel pipes. This in general comprises,
(i) carrying out edge preparation on the steel plate, PA0 (ii) performing an edge-bending process on the edges of the plate, by means of a crimping press, PA0 (iii) forming the plate into U-shape through an U-ing process, PA0 (iv) performing an O-ing process on the U-shaped pipe-blank, PA0 (v) subjecting the O-shaped pipe-blank to tack welding at butted edges, PA0 (vi) subjecting the tack welded pipe to an inner surface welding and an outer surface welding, and PA0 (vii) expanding the pipe by means of for example, a mechanical expander.
The UOE process has been employed in the production of steel pipe of large diameter due to its characteristics. However, a thick wall and high strength are required for deep-sea pipeline or structural steel pipe, and a big problem occurs in the UOE process in producing thick wall steel pipe of large diameter which is more than 2% in thickness/outer diameter. More particularly, peaking is inevitably caused.
The term "peaking" means a deviation of the butted edge from the outer diameter, i.e., a degree defined by delta in FIG. 1 (projecting from the regular circle Q). The peaking creates inconveniences such as instability at welding after the O-ing which causes defects in the weld. Further, the peaking remaining after the weld generates large angular distortion on the seam part during the expansion process and may bring about an expansion crack. Furthermore, even in the product, stress is centralized on the welded part owing to the inner load in use.
Therefore, in the UOE pipe production process, this peaking should be decreased as much as possible before the weld. For removing the peaking, a process utilizing edge-bending by a crimping press is considered. However, this process depends upon the bending moment M.sub.0 =F.multidot.L between two points F and F as shown in FIG. 2. In order to bend the vicinity of the edges (L.fwdarw.0), a load F obtaining the constant moment M.sub.0 becomes infinite theoretically. Therefore 1.0 to 1.5 t (t=thickness) from the edge of the plate generally remains as non-processed, i.e., straight.
FIG. 3 shows the peaking after O-ing for a pipe which has been subjected to the edge-bending by means of a crimping press of 1500 t. It is noted that the higher becomes the peaking, the higher are the thickness and the strength of the pipe ("X65" and "X42" mean the strength grade of the pipe). Therefore, only using the crimping press is not enough to reduce the peaking.
As a method of reducing the peaking, the edge-bending process has been tried. According to this process, illustrated in FIG. 4, since the steel plate is effected with compressive stress in the circumferential direction at pressing by means of an upper die A and a lower die B, the peaking is more or less decreased during the compressing step. However the edge-bending by O-ing is a kind of buckling phenomena as shown in FIG. 4, and a distance L giving the moment between fulcra is small and the efficiency is inferior. Therefore, a great pressing load is required to reduce the peaking by O-ing. That is, for reducing the peaking on the steel pipe in conditions of API grade, X65, t/D&gt;5% and 12 m length of the pipe, a pressing power of more than 60,000 or 70,000 t is required. But an apparatus generating such power is large scaled and difficult in view of the building, and besides is very expensive.
Based on these difficulties, a method has been considered whereby the O-shaped pipe-blank is, after O-ing but prior to welding, reduced in circumference by an up-set shrinker. This method reduces in diameter the O-shaped pipe-blank within a possible range from the outer face by means of the up-set shrinker in reverse to the expansion after the welding. In such a way, the vicinity of the butting edges is effected with edge-bending through compression in the circumferential direction, thereby to decrease the peaking. In a case of such a method, the diameter of the pipe is reduced in general by around 1 m.
However, since this method merely reduces the blankwork from the overall circumference by means of the shrinker dies surrounding the pipe, the mechanism of decreasing the peaking is not different in substance from the buckling phenomena by O-ing, due to which efficiency is not sufficient in reducing the peaking, and the extreme compression plastic deformation is imparted to the entire pipe for the purpose of reducing the peaking. As a result, the strength is lowered by the Bauschinger effect and the toughness is deteriorated by the plastic deformation.
It is an object of the invention to provide an up-set shrinker which is able to control the peaking to be at a minimum on the butted edges in the reducing process after the welding.
It is another object of the invention to provide an apparatus which is able to control the peaking considerably with a low forming load without imparting the extreme compression plastic deformation to the overall blankwork.
It is a further object of the invention to provide an apparatus which is able to carry out the process in accordance with the strength and the thickness of the material, thereby to reduce to a minimum the peaking for material of any strength and thickness.
It is a still further object of the invention to provide an apparatus which is able to greatly reduce the peaking of an O-shaped pipe-blank.
It is another object of the invention to provide an economical apparatus which is simple in structure and easy to manufacture.